1. The field of the art
The present invention relates to a circuit for converting a binary dither image to a multilevel image.
2. The background of the art
International standardization for step by step transmission is governed at the present time by the CCITT (Comite Consultatif International pour Telegraphie et Telephonie) and the ISO (International Standards Organization).
A method for realizing step by step transmission of a multilevel image was described in 1987 by Kim, Kato, and Sita in "A step by step transmission of an intermediate tone image using a binary value assumption of multilevel image" Electronics Information & Communication Society Journal. This article describes a method of dither processing an original image to obtain a dither image.
In this process, tone information is obtained using the dither image and forming a multilevel image from the dither image. The multilevel image is compared to the original signal to obtain a difference signal indicating one of many grey levels between absolute white and absolute black. Thus, the transmission of the difference signal and the dither image allows transmission of a overall data signal of much smaller size and equivalent high quality than a comparable signal transmission not using a dither image process.
As shown in FIG. 1, an original image is dither-processed and transmitted. This dither image is also converted into a multilevel image and a difference between the multilevel image and the original image is encoded and transmitted. On the receiving side, the multilevel image is reproduced using the same algorithm as on the sending side. The reconstructed difference component and the reconstructed multilevel image are then summed to reproduce the original image.
This method is problematical because the software processing techniques to implement the above-mentioned conversion method are slow.